Chapter 3: Digestion
Digestive System
Essentially a hollow tube extending from mouth
to anus
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Epithelium lines the lumen
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Barrier to invaders
Submucosal layer
Muscularis
4 sphincters (valves) located in certain locations
to prevent food/fecal matter from backing up
Under autonomic (automatic) control
Fig. 3.2
GI Tract
Food
Bolus
Chyme
“Waste”
Mouth
Esophagus
Stomach
Duodenum
Jejunum
Ileum
Cecum
Ascending colon
Transverse colon
Descending colon
Sigmoid colon
Movement Along the Intestine
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Peristalsis
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Segmentation
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A ring of contraction propelling material along
the GI tract
A back-and-forth action that breaks apart food
Mass movement
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Peristaltic wave that contracts over a large area of
the large intestine to help eliminate waste
Movement
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Peristalsis
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Segmentation
4 Sphincters
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Cardiac or lower esophageal sphincter
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Pyloric sphincter
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Controls the amount of stomach content into the small
intestine
Sphincter of Oddi
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Prevents reflux of stomach content to cause heartburn
and ulcers
Controls the amount of bile into the small intestine
Ileocecal sphincter
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Prevents large intestine content (bacteria) back up into
the small intestine
An Example of a Sphincter Muscle
Esophagus
Circular muscle
Longitudinal muscle
Esophagus muscles relax,
opening the passageway.
Stomach
Diaphragm muscles relax,
opening the passageway.
Esophagus muscles contract,
squeezing on the inside.
Diaphragm muscles contract,
squeezing on the outside.
When the circular muscles of a sphincter contract, the
passage closes; when they relax, the passage opens.
Mouth to Stomach
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Cooking actually starts the digestion
Chewing
Saliva (3 functions)
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Contains enzymes to help breakdown carbohydrates
Provides mucus to lubricate the food for easier
swallowing
Contains lysozymes to kill bacteria
Tongue
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Taste receptors
Esophagus
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Food propels down by rhythmic muscle contractions
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Epiglottis functions to cover the passage to the
airway and prevent food from entering the lungs
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strong enough that even food swallowed while standing
on your head will reach your stomach
bacteria naturally present in food can cause pneumonia
Ends at cardiac or lower esophageal sphincter
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prevents reflux of stomach content that cause heartburn
and ulcers
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The Stomach
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Capacity of ~4 cups
Secretion of hydrochloric acid and enzymes
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begins digestion of proteins
Mucus layer prevents autodigestion from acid/enzymes
Holds food for 2-6 hours
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Passage into small intestine regulated by pyloric sphincter
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Distension of small intestine inhibits empyting
Large meal takes longer to leave the stomach
Solid meal takes longer than liquid
More complex meal takes longer
Higher fat meal takes longer
Secretion of the intrinsic factor
Physiology of the Stomach
How does the stomach know to
produce acid?
Stimulated by
 Stomach distention
 Histamine
 Thoughts of food (nerve input)
 Food itself
 Hormone: Gastrin
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Additional Function
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Assists in calcium absorption
The pH Scale
pH’s of common substances:
Basic
14
Concentrated lye
13
Oven cleaner
12
11
Household ammonia
10
6
Baking soda
Bile
Pancreatic juice
Blood
Water
Saliva
Urine
5
Coffee
4
Orange juice
3
Vinegar
2
Lemon juice
Gastric juice
9
8
pH neutral
7
1
Acidic
0
Battery acid
The Small Intestine
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Duodenum
 ~10 inches in length
 Primary site of digestion
Jejunum
 ~4 feet in length
 Some digestion
Ileum
 ~5 feet in length
 Little digestion
Activity In the Small Intestine:
Accessory Organs
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Bile acid from the liver via the gallbladder
Bicarbonate ions and enzymes from the
pancreas
Muscle contractions to mix the food with
digestive juices
Food remains 3-10 hours in the small intestine
~95% of digestion takes place here
The Small Intestine
Anatomy of the Small Intestine
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The wall is folded
Villi projections are located on the folds
Microvilli is located on the villi
Glycocalyx is located on the microvilli
Increases intestinal surface area 600 x
Absorptive cells (enterocytes) are located on
the villi
Intestinal Mucosa
Absorptive cells
 Produced in crypts
 Migration and maturation from the crypts to the
tips of the villi
 Degradation of cells at the tips of the villi by
digestive enzymes
 Newly formed cells constantly migrate to replace
dying ones (3-6 days)
 High turnover causes the cells to deteriorate
during nutrient deficiency
Gastrointestinal Hormones
Gastrin
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Originated from the pyloric region of the
stomach and upper duodenum
Stimulated by food, thoughts of food
Stimulates flow of stomach enzymes and HCl
Stimulates contraction of cardiac sphincter
Slows gastric emptying
Secretin
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Originated from the duodenum, jejunum
Stimulated by the presence of acidic chyme
and the presence of peptones in the duodenum
Stimulates the secretion of bicarbonate
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Neutralizes stomach acid
Slows gastric emptying
Cholecystokinin (CCK)
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Originated from the duodenum, jejunum
Stimulated by food, presence of fat and
protein in the duodenum
Stimulates contraction of gallbladder and flow
of bile
Stimulates the release of enzyme rich
pancreatic fluids
Slows gastric emptying
Gastric Inhibitory Peptide (GIP)
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Originated from the duodenum, jejunum
Stimulated by fats and protein
Inhibits the secretion of stomach acid and
enzymes
Slows gastric emptying
Site of Absorption
Types of Absorption
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Passive
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Facilitated
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A carrier shuttles substances into the absorptive cells
Going from higher to lower concentration
No energy expended
Active
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Intestinal wall is permeable to the nutrient
Going from higher to lower concentration
No energy expended
Uses a carrier and ATP
Endocytosis
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Phagocytosis and pinocytosis
Types of Absorption
After Absorption: Circulation
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Intestinal villi drains into
1. Portal circulation
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Water-soluble vitamins and minerals
Monosaccharides and amino acids
Portal vein
2. Lymphatic circulation
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Fat-soluble
Large particles
Thoracic duct
Left subclavian vein
The Large Intestine
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Little digestion occurs
Indigestible food stuff (ie fiber)
Absorption of 85-90% of the water,
some minerals, vitamins
Formation of feces for elimination
Colon Health
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Colon contains both good and bad bacteria (called
microflora)
Usually exist in balance
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Imbalances can cause problems
Good bacteria have beneficial effects on the colon
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Protect against invading pathogenic bacteria
Synthesize vitamin K
produce short chain fatty acids
Probiotics
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Consumption of beneficial bacteria in foods
or supplements
Yogurt with “live and active cultures”
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L. acidolphilus and Bifidus
Other claims:
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prevents diarrhea
boosts the immune system
supresses some cancers
Lowers cholesterol and blood pressure
Prebiotics
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Consumption of foods that promote growth of
good bacteria (and not the bad bacteria)
Consist of non-digestable food ingredients
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Examples: banana, beans
Active ingredients are fructo-oligosachhrides
(FOS)
Available in supplemental form
A Summary
ORGAN
FUNCTIONS
Mouth
Chewing, digestion of starch
Esophagus
Passage way
Stomach
Food Storage, acid kills bacteria
Some protein digestion
Small Intestine (duodenum, jejunum Final digestion
and ileum)
Absorption of most of the nutrients
Large Intestine
Absorption of water, some minerals,
fatty acids
Rectum
Elimination
Liver
Production of bile
Gallbladder
Store and release bile
Pancreas
Enzymes and bicarbonate
Gastrointestinal Tract-Recap
GI Problems
Dysphagia
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Difficulty swallowing
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Causes: usually a stroke with subsequent
paralysis
Can be temporary or permanent
Risk for aspiration
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Food entering the lungs
Swallow study will determine if at risk
Diet therapy may involve liquid, thickened
liquid or pureed foods.
Normal Swallowing and Choking
Tongue
Food
Larynx rises
Epiglottis closes
over larynx
Esophagus (to stomach)
Trachea (to lungs)
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Heartburn (GERD)
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Acid from the stomach to the esophagus due to relaxing of
esophageal sphincter
Symptoms: Gnawing pain in the upper chest
Causes: obesity, pregnancy
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Diet Treatment
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Smaller, less fatty meals, chew thoroughly, eat slowly
Do not lie down after eating, don’t exercise for 2 hours
Save drinking fluids for between meals
Avoid general list of offending foods
Chocolate, caffeine, peppermint, spearmint, onions garlic, peppers,
spicy and greasy foods, acidic foods
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Gastroesophageal Reflux (GERD)
Esophagus
Reflux
Diaphragm
Weakened lower
esophageal
sphincter
Acidic stomach
contents
Stomach
GERD/Esophageal stricture
Barrett’s esophagus
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develop as a result of long term exposure to
stomach acid
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Normal cells that line the esophagus are replaced
by different types of cells
Can develop into esophageal cancer
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Ulcers (stomach or small intestine)
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Open sore in lining of stomach or small intestine
Symptoms: pain 2 hours after eating, weakness,
anemia, black or bloody stools
Cause: Helicobacter pylori, heavy use of aspirin,
excessive acid production in the stomach
Diet Treatment: nothing specific, avoid offending
foods, limit caffeine, avoid alcohol, smaller meals
Fig. 3.14
Gallstones
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Precipitation of cholesterol, bile and calcium
into stones in the gall bladder
Symptoms: pain after eating
Cause: stones caught in bile ducts
Diet Treatment: avoid greasy foods
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many can even tolerate greasy foods
Celiac Sprue (gluten intolerance)
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genetic disorder where eating certain types of
protein, called gluten, sets off an autoimmune
response that causes damage to the small intestinal
villi. This, in turn, causes the small intestine to lose
its ability to absorb the nutrients.
Symptoms: chronic diarrhea, abdominal pain,
malnutrition
Cause: genetic, but usually requires a trigger
Diet treatment: complete avoidance of barley,
wheat and rye
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Normal vs. Celiac
Constipation
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Slow movement of fecal matter which increases
fluid reabsorption causing hardening of the feces
Symptoms: abdominal distention, pain, discomfort
Causes:
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Results from ignoring normal urge
Antacids, calcium and iron supplements
Lack of fiber in diet, sedentary lifestyle
Diet Treatment
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Plenty of dietary fiber and fluids
Foods with laxative type effect: prune juice
Diarrhea
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Medical definition: For people in the Western
World, the usual amount of water in stool
each day is generally no more than 200 ml or
7 oz. (8 oz. = 1 cup). When it is consistently
more than this, it is called diarrhea.
Causes: stress, bacteria, certain foods,
prescription drugs
Diet Treatment: no specific, focus on
rehydration
Hemorrhoids
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Swollen veins of the rectum and anus
Symptoms: pain, bleeding, itching and irritation
Cause: added stress and pressure to the vessels due
to poor bowel habits, constipation, diarrhea,
pregnancy, obesity, and especially frequent straining
when having a bowel movement.
Diet Treatment
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Adequate fiber and fluid
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Irritable Bowel Syndrome (IBS)
IBS = Crohn’s disease and/or Ulcerative Colitis
3 factors:
1. Altered intestinal motility
2. Increased intestinal sensitivity (abdominal pain)
3. likely due to communication issues between digestive tract
and the brain
Majority of cases women in 20’s and 30’s, may have a genetic
factor, but 2nd insult or injury must occur
Various severities: 25% severe/ 5% very severe
IBS (Irritable Bowel Syndrome)
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Ulcerative Colitis
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chronic, recurring disease of the large intestine
Symptoms: abdominal pain, diarrhea, bleeding
Cause: unknown, however may be a defect in the
immune system in which the body's antibodies
actually injure the colon. Also could be an
unidentified microorganism or germ is responsible
for the disease. Likely a combination with genetics.
Diet Treatment: nothing specific, avoid foods that
seem to cause more cramping and diarrhea
Limit or avoid caffeine
Crohn’s Disease
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Chronic, recurrent inflammatory disease of the intestinal tract
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Symptoms: abdominal cramps, diarrhea
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Long term: anemia and weight loss
Other characteristics
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Can occur in both small and large intestine
Begins in teens and twenties
Cause: unknown, however genetics and an immune response
are possible factors
Diet Treatment: nothing specific, avoid foods that seem to
cause more cramping and diarrhea
Limit or avoid caffeine
Crohn’s
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Diverticulosis
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Pouches or bubbles that protrude out from the colon
wall due to extra pressure
Symptoms: severe abdominal pain which can
eventually cause total obstruction, bleeding
Causes: low fiber diets
Diet Treatment:
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Short-term: low fiber diet
Long-term: follow a high fiber diet, avoid foods with little
seeds and husks
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Strawberries, corn, popcorn, nuts
Diverticula in the Colon
Diverticula (plural)
Diverticulum (singular)
Diverticula may develop anywhere along the GI tract,
but are most common in the colon.
Diverticulosis/Diverticulitis
Colorectal Cancer
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Uncontrolled cell division that evolve into abnormal
cells
Symptoms: blood in stool, change in stool,
abdominal pain, fatigue
Causes: genetics, high fat, low fiber diet, excessive
red meat
Diet Treatment: Diet high in fruits and vegetables
and avoid excessive amounts of red meat
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diet has more of a preventative affect